1. Field of the Invention
The present invention relates to an optical receiver, and particularly, to an optical receiver by which a DQPSK-modulated optical signal is demodulated to a multilevel phase-modulated signal.
2. Description of Related Art
As communication traffic increases, a multilevel modulation/demodulation coding technology is under consideration for a next-generation long-range optical communication system demanding high speed and high capacity. As a representative example, a differential quadrature phase shift keying (DQPSK) scheme has been introduced. The DQPSK scheme has a narrower signal frequency band in comparison with an existing on-off keying (OOK) scheme. In addition, it is anticipated that it is possible to improve frequency efficiency, extend transmission distance, or obtain high sensitivity.
First, in the quadrature phase shift keying (QPSK) modulation scheme, θ, θ+π/2, θ+π, and θ+3π/2 are allocated to each symbol 00, 01, 11, and 10 obtained from two-bit data, where θ denotes any phase. A receiver recovers transmission data by detecting a phase of the received signal. As a method of relatively readily implementing the QPSK modulation scheme, a DQPSK modulation scheme has been introduced, in which a phase-change amount (0, π/2, π, and 3π/2) of a carrier wave between a symbol value previously transmitted and a symbol value subsequently transmitted corresponds to two bits of the transmission information. Therefore, the receiver can recover transmission data by detecting a phase difference between two contiguous symbols.
As disclosed in Japanese Unexamined Patent Application Publication Nos. 2006-295603 and 2007-158852, in order to demodulate the DQPSK-modulated optical signal, it is necessary to provide two delayed interferometers for generating an I (In-phase) signal and a Q (Quadrature) signal (in other words, a delayed interferometer of waveguides 102 and 103 or a delayed interferometer of waveguides 104 and 105) shown in FIG. 1, and demodulate the phase difference with a high accuracy. In FIG. 1, the optical signal beam α is branched into two light beams by a branch unit 101 in order to input them to two delayed interferometers. However, due to influences from the atmospheric temperature of the optical receiver, an optical path length within the delayed interferometer is changed, and the phase is hardly stabilized, which makes it difficult to achieve a high-accuracy demodulation. In addition, it may be impossible to identify which interferometer demodulates which signal component. It is also necessary to optimally adjust a difference between the optical path lengths after the optical signal is branched into two light waves until arriving at the two interferometers or a difference of the optical path lengths included in each interferometer. This may make a control system significantly complicated. Furthermore, the I or Q signal can be obtained by inputting light waves a1 and a2 or a3 and a4 into corresponding balanced photo-sensitive elements.
In order to solve the aforementioned problems, in Japanese Patent Application No. 2008-255528, the applicant proposed an optical receiver based on a polarization surface as shown in FIG. 2. Specifically, the DQPSK modulation light beams a are incident to a one-bit delay circuit with the polarization surfaces being aligned in a single direction. The one-bit delay circuit includes polarization-maintaining type fiber couplers 1 and 2 and a half wavelength plate 3 which rotates the polarization surface by 90° across the path of one of the branched light beams. As a result, the multiplexed light waves are synthesized such that the polarization surfaces of two signal light beams delayed by one bit are perpendicular to each other.
Then, the two signal light beams are separated into four signal light beams using a polarization separation circuit 4, and, for example, light waves b1 and b2 or b3 and b4 are incident to corresponding balanced photo-sensitive elements, thereby obtaining the I or Q signal. As a result, the one-bit delay circuits can be integrated into a single one, and it is possible to reduce manufacturing cost and awkwardness when adjusting optical components.
However, if the one-bit delay circuit is manufactured using, for example, a polarization-maintaining type fiber coupler, which is very sensitive to disturbances such as a temperature change or an external stress, device stability may be insufficiently guaranteed. Also in a case where a spatial optical system such as a half mirror, which has a polarization dependency, is employed, error may be easily generated across the optical path length of the branched light wave, and it is highly probable that a temporal difference will be generated between the I and Q component signals.